Hitherto, a technology of adding sensitizing dye(s) to a silver halide emulsion to enlarge the sensitive wavelength range of the emulsion, so as to optically sensitize it, has been well known in the field of silver halide photographic materials.
Many compounds have heretofore been known as color sensitizing dyes to be used for this purpose, for instance, the cyanine dyes, merocyanine dyes, xanthene dyes and others described in T. H. James, The Theory of the Photographic Process, 3rd Ed., 1966 (published by Macmillan Co., New York), pages 198 to 228.
Where such sensitizing dyes are applied to silver halide emulsions, in general, they must not merely enlarge the sensitive wavelength range of the silver halide emulsions but also must satisfy the following conditions:
(1) they have a suitable color-sensitizing range; PA1 (2) they have a high sensitizing efficiency and may yield a sufficiently high sensitivity; PA1 (3) they do not cause fogging; PA1 (4) they may sensitize silver halide emulsions in such a way that the sensitivity of the sensitized emulsion does not fluctuate much under variation of the ambient temperature during exposure; PA1 (5) they do not have any bad interaction with other additives such as stabilizers, antifoggant, coating aids and couplers; PA1 (6) where silver halide emulsions to which sensitizing dyes have been added are stored, the sensitivity of the emulsions does not fluctuate; in particular, where they are stored under high temperature and high humidity conditions, they are free from fluctuation in the sensitivity thereof; and PA1 (7) the sensitizing dyes added to silver halide emulsions do not diffuse into any other light-sensitive layers to cause color mixing after development.
The above-mentioned conditions are important in preparing silver halide emulsions for silver halide photographic materials.
However, despite of various trials and attempts, lowering of the sensitivity of raw films during their storage could not be prevented to a satisfactory degree.